1. Field of the Invention
The present invention generally relates to formation of copper interconnects or copper wiring filled grooves such as contact holes or via holes, which is suitable for manufacturing fine structure semiconductor devices.
2. Description of the Related Art
In next-generation wiring structures, contact holes that connect diffusion layers and wiring layers on a silicon substrate, and via holes that connect metal wirings, are connection holes for connecting wirings. As semiconductor devices become increasingly finer, aspect ratios of these connection holes are also increasing and concerns are raised over the rise in contact resistance and via resistance. As for contact resistance, tungsten-filled structures have been traditionally used. To address fine contact holes, a method to form a Ti/TiN CVD film and then fill the holes with a W film using CVD has been used. However, the high resistivity of this W film causes the W plug resistance to increase when the contact hole diameter becomes 50 nm—a size required of devices at the 32-nm technology node generation and beyond, and the aspect ratio reaches 5 or more. The higher W plug resistance can inhibit device performance (V. Arnal, A. Farcy, M.Aimadeddine, V. Jousseaume, L. G. Gosset, J. Guillan, M. Assous, L. Favennec, A. Zenasni, T. David, K.Hamioud, L-L. Chapelon, N. Jourdan, T. Vanypre, T. Mourier, P. Chausse, S. Maitrejean, C. Guedj, J. Torres, “Materials and processes for high signal propagation performance and reliable 32 nm node BEOL”, pp.1-3, International Interconnect Technology Conference, IEEE (2007)).
For this reason, use of copper, Rh or other low-resistance material to fill contact holes is proposed (Shao et al., Proceedings of IITC 2007, pp. 102-104). If a Cu contact plug is used, a copper-diffusion barrier film is needed to prevent diffusion of Cu into the diffusion layer, and a seed layer must also be formed that can form copper in a manner achieving good adhesion while offering good coatability to ensure favorable filling property. Methods to fill contact holes include one that uses plating and another that uses chemical vapor deposition. Under the plating method, a seed layer having favorable coatability is required, along with deposition of an atomic layer to form this seed layer, or a Cu seed layer or connection seed layer constituted by Ru, etc., formed by chemical vapor deposition. On the other hand, another method is reported whereby contact holes are filled with copper by means of chemical vapor deposition using an underlayer constituted by a Ru or other film that allows for easy formation of a core to achieve chemical vapor deposition of copper.
It is becoming increasingly difficult to form a Cu-diffusion barrier layer or Cu seed layer over via holes of high aspect ratios using the conventional PVD. Particularly with a Cu-diffusion barrier film by PVD, forming a thin, continuous Cu-diffusion barrier film is difficult on the side face at the bottom of the via hole, and therefore formation of such film using a method that can achieve favorable coatability is desired. Similarly when a Cu seed layer is formed over via holes of high aspect ratios using the conventional PVD, the Cu film tends to agglomerate on the side face at the bottom of the contact or via hole, and the film tends to become discontinuous as a result. Accordingly, a method to form RuTa alloy beforehand to facilitate formation of PVD-Cu is suggested, as proposed in US2007/0059502 A1. In this case, RuTa alloy with a Ru composition ratio of 80% or more is formed as the underlayer for Cu seed layer to form a continuous Cu film as a Cu seed film. However, although introduction of Ru certainly improves the morphology of the PVD-Cu film, forming a Cu film with good coatability through PVD-Cu is also becoming difficult as the trend for finer via holes accelerates.
Accordingly, a method is proposed whereby, instead of forming a Cu film by PVD, a film constituted by Ru, for example, is formed and after a Ru film has been formed, the Ru film is used as a seed layer for Cu electrolytic plating without forming a Cu seed layer (“PEALD of Ru layers on WNC ALD barrier for Cu/porous low-k integration,” Proceedings of Advanced Metallization Conference 2006, p. 39). In this case, the Ru film can be formed by CVD, ALD or other method capable of achieving good coatability, which solves the coatability problem presented by PVD. On the other hand, the Ru film thickness must be increased to at least approx. 5 to 10 nm to form a seed layer whose resistance is low enough to support electrolytic plating. Considering that next-generation Cu wirings require via holes of approx. 30 nm in size, therefore, 10 to 20 nm of the 30-nm hole will be occupied by a Ru film. On the other hand, the resistivity of Ru film is one digit higher than that of Cu film, which leads to higher via resistance and contact resistance.
If a thin Ru film can be formed, it is possible to form a Cu seed film by CVD (chemical vapor deposition) or ALD after forming a thin Ru film. To achieve a continuous Cu seed film, however, the film thickness must be at least 5 to 10 nm. Let's assume that the via hole size is 30 nm and a Cu barrier film is formed by 2 nm below a Ru film. Even if the Ru film is as thin as 2 nm, the Cu seed film is at least 5 nm thick and therefore the total thickness becomes 9 nm. As a result, 18 nm of the via hole diameter of 30 nm will be occupied by the Cu barrier film, Ru film and Cu seed film. In electrolytic plating where the via hole height is 200 nm, therefore, the aspect ratio will become 200 nm/12 nm=16. This aspect ratio is extremely tight for electroplating, even with electrolytic plating. Resolving this problem in the real world using existing electrolytic plating technology is extremely difficult.
In light of the above, forming Cu wirings in connection holes using the conventional PVD alone is becoming increasingly difficult in the formation of next-generation Cu wirings involving higher aspect ratios. Also, application of a Ru-type film presents the problem of increasing wiring resistance in connection holes. Furthermore, if a thin Ru-type film is applied, the aspect ratio of connection holes becomes too high for electrolytic plating. If a thin Ru-type film is applied and connection holes are filled by a Cu film by means of chemical vapor deposition, seams and voids tend to occur.
The above mainly described the formation of Cu plugs in contact holes, but next-generation semiconductor devices require new technology to fill Cu not only in connection holes provided in diffusion layers on a silicon substrate, but also in connection holes of high aspect ratios of around 5 to 15 used to connect the W wirings in the lower layer and Cu wirings in the upper layer. With respect to such technology, the Cu volume must be increased as much as possible if a thin Cu barrier film is to be formed in a connection hole of a high aspect ratio to fill the hole while reducing the via resistance at the same time. For this reason, a very thin film offering good coatability and excellent adhesion with Cu must be formed as a Cu barrier seed.